The invention relates to lighting fixtures and, more particularly, to luminaires that are well-suited for use with solid state lighting sources, such as light emitting diodes (LEDs).
Lighting devices are ubiquitous in residential, commercial, office and industrial spaces throughout the world. More recently, with the advent of efficient solid state lighting sources, these lighting devices have been used with LEDs as the light source. LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED. The light output intensity is typically directly proportional to the forward current flowing through the LEI).
Heat may adversely affect the operation of a LED lighting device. Excessive heat may degrade the performance of, or cause a failure of, the LEDs or other components including the LED driver. Thermal management is used with solid state lighting devices to manage the heat generated in the system. To manage heat, LED lighting devices may use passive heat management systems such as mechanical heat sinks, gas convection or the like. Such passive systems may not be capable of dissipating enough heat from the lighting device under all conditions. In response, active heat management systems have been developed. One such system uses a circuit on the LED board that includes a thermistor. The thermistor provides a resistant load that sends a feedback signal to a dimmer circuit for the LEDs. As the set temperature limit is reached, the dimmer circuit dims the LEDs by effectively lowering the current delivered to the system thereby reducing the heat generated. The LEDs are dimmed until the thermistor drops below the set temperature limit. The cycle is repeated to maintain the temperature below the set temperature limit. The use of a thermistor on the LED board and the feedback circuit to the dimmer circuit requires more system components thereby increasing the cost of the system and lowering reliability. LED systems may also use driver temperature limits to protect the driver from overheating. In such a system the driver itself includes a temperature sensor and feedback that reduces power to the LEDs when the temperature limit of the driver is reached. While such a system protects the driver against overheating it does not necessarily protect other components in the system from overheating.